The transmission of pathogenic Leishmania involves an injection of extracellular promastigotes into a mammalian host by an infected sandfly. The promastigotes rapidly attach and enter monocytes and cells of the reticuloendothelial system, where they transform into amastigotes and multiply within phagolysosomes. Analysis of the interaction of Leishmania promastigotes with the target host cell suggests that both parasite and host molecules are involved in cell adhesion. Clinical symptoms of leishmaniasis range from self-healing skin lesions to diffuse cutaneous and mucosal manifestations, or severe visceral involvement of the spleen, liver and lymph nodes (visceral leishmaniasis or VL).
Visceral leishmaniasis is generally caused by Leishmania donovani in Africa and India, L. infantum in Southern Europe or L. chagasi in Latin America. In VL, high levels of parasite specific antibodies are observed prior to detection of antigen specific T cell responses (Ghose et al., Clin. Exp. Immunol. 40:318-326, 1980). This antibody response has been exploited for serodiagnosis of infection with L. chagasi and L. donovani. The current World Health Organization's estimate of 12 million cases of leishmaniasis and recent epidemics of visceral leishmaniasis in Sudan and India highlight the need for more effective early diagnosis and therapeutic agents. Also at least 400,000 new cases of VL are diagnosed annually. The current diagnostic tests to measure an antibody response use whole or lysed parasites. Therefore, there is a need in the art to improve the diagnostic accuracy for diagnosing VL early while the potentially fatal disease is more treatable.
Recovery from leishmaniasis correlates with the development of specific T lymphocyte responses and usually confers long-lasting immunity against reinfection (Carvalho et al., J. Clin. Invest. 76:2066-6, 1985 and Carvalho et al., J. Immunol. 135:4144-8, 1985). Both recovery from disease and resistance to reinfection are dependent on the development of specific T lymphocyte responses. Interferon gamma (IFN-.gamma.) is a product of activated T cells, has demonstrated anti-leishmania activity in vitro (Murray et al., J. Clin. Invest. 72:1506, 1983 and Nacy et al., J. Immunol. 135:1305, 1985), and in vivo (Reed et al., J. Immunol. 132:3116, 1984 and Murray. et al., J. Immunol. 1348:2290, 1987) and has been used effectively in the clinical treatment of leishmaniasis (Harms et al., Lancet 10:1287, 1989 and Badaro et al., N. Engl. J. Med. 322:16, 1990).
One antigen, called gp63, has been cloned (Miller et al., Mol. Biochem. Parasitol. 38, 267-274, 1990) and was found to be a metalloprotease and is highly conserved among different species of Leishmania (Etges et al., J. Biol. Chem. 261:9098, 1986; Chaudhuri et al., Mol. Biochem. Parasitol. 27:43, 1988; Chaudhuri et al., J. Biol. Chem. 264:7483, 1989; Colmer-Gould et al., J. Exp. Med. 162:902, 1985; and Button et al., J. Exp. Med. 167:724, 1988). Gp63 is relatively abundant on both the infective promastigote stage and the intracellular amastigote stage (Frommel et al., Mol. Biochem. Parasitol. 38:25-32, 1990 and Medina-Acosta et al., Mol. Biochem. Parasitol. 37:263, 1989). Gp63 is important for both parasite entry into macrophages (Russel and Wilheim, J. Immunol. 136:2613, 1986; Chang et al., Proc. Natl. Acad. Sci. USA 83:100, 1986; Wilson and Hardin, J. lmmunol. 141:265, 1988; and Mosser and Edelson J. Immunol. 135:2785, 1985) and subsequent survival within the phagosome (Chaudhuri et al., J. Biol. Chem. 264:7483, 1989). Immunization with native gp63 in vivo partially protected susceptible mice against cutaneous disease (Handman and Mitchell Proc. Natl. Acad. Sci. USA 82:5910, 1985 and Russel and Alexander J. Immunol. 140:1274, 1988). Moreover, recombinant gp63 expressed in Salmonella conferred partial protection by oral immunization against Leishmania major infection in resistant mice (Yang et al., J. Immunol. 145:2281, 1990). Both native gp63 and recombinant gp63 elicited strong proliferative responses, as well as IFN-.gamma. production, from leishmaniasis patients with a spectrum of clinical disease (Russo et al., J. lmmunol. 147:3575, 1991).